Hello Marko
As I’m a heavy Proxmox user, maybe I can help you with your Proxmox Interface issues.
Is the Proxmox running at home, or at a Provider (hosted)?
Most likely are two issues:
Proxmox prefers a connected NIC, at a hosted environment, usually only one is actually connected (=active).
The second is a misconfiguration of the 2nd NIC in Proxmox, often seen when using DSL…
My 2 cents
Andy